This invention relates to an energy efficient process to produce viscous oil from a subsurface formation. More specifically, electricity produced by burning combustible material and hot water produced by heat exchange with the hot flue gases are combined to heat and pressurize the formation in an energy efficient manner.
Large deposits of viscous hydrocarbonaceous substances are known to exist in subterranean formations such as for example, the Ugnu formation in Alaska. Many techniques have been proposed for producing oil from formations containing viscous oils. One such proposal uses steam to heat and force oil out of the formation; but steam at the pressure levels of interest requires steam temperatures of about 500 to 600.degree. F. For the most part, calculations indicate that the major portion of the benefit obtained from heating a formation will be achieved at temperatures of between 150.degree. and 200.degree. F. Steam temperatures of 500.degree. to 600.degree. F., therefore, use heat inefficiently.
It also has been proposed, for example, in U.S. Pat. Nos. 3,642,066; 3,874,450; 3,848,671, 3,948,319; 3,958,636; 4,010,799 and 4,084,637, to use electrical current to add heat to a subsurface pay zone containing tar sands or viscous oil to render the viscous hydrocarbons more flowable. Two electrodes are connected to an electrical power source and are positioned at spaced apart points in contact with the earth. Currents up to 1200 amperes are passed between the electrodes. Electrical heating processes seem to be consistent with temperatures of between 150.degree. and 200.degree. F., but power efficiency of electrical generation is only about one third. When the power is generated by burning a combustible material, e.g. methane or oil produced in the area of the viscous oil formation, in essence the combustible fuel is traded for electrical power which is then traded for viscous oil.
It is the primary object of this invention to provide a more energy efficient viscous oil recovery process operating within the optimum temperature beneficial range.